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We report the synthesis and reactivity of a model of [Fe]-hydrogenase derived from an anthracene-based sca ﬀ old that includes the endogenous, organometallic acyl(methylene) donor. In comparison to other non-sca ﬀ olded acyl-containing complexes, the complex described herein retains molecularly well-de ﬁ ned chemistry upon addition of multiple equivalents of exogenous base. Clean deprotonation of the acyl(methylene) C – H bond with a phenolate base results in the formation of a dimeric motif that contains a new Fe – C(methine) bond resulting from coordination of the deprotonated methylene unit to an adjacent iron center. This e ﬀ ective second carbanion in the ligand framework was demonstrated to drive heterolytic H 2 activation across the Fe( II ) center. However, this process results in reductive elimination and liberation of the ligand to extrude a lower-valent Fe – carbonyl complex. Through a series of isotopic labelling experiments, structural characterization (XRD, XAS), and spectroscopic characterization (IR, NMR, EXAFS), a mechanistic pathway is presented for H 2 /hydride-induced loss of the organometallic acyl unit ( i.e. pyCH 2 – C ] O / pyCH 3 +C ^ O). The known reduced hydride species [HFe(CO) 4 ] (cid:1) and [HFe 3 (CO) 11 ] (cid:1) have been observed as products by 1 H/ 2 H NMR and IR spectroscopies, as well as independent syntheses of PNP[HFe(CO) 4 ]. The former species ( i.e. [HFe(CO) 4 ] (cid:1) ) is deduced to be the actual hydride transfer agent in the hydride transfer reaction (nominally catalyzed by the title compound) to a biomimetic substrate ([ Tol Im](BAr F ) ¼ ﬂ uorinated imidazolium as hydride acceptor). This work provides mechanistic insight into the reasons for lack of functional biomimetic behavior (hydride transfer) in acyl(methylene)pyridine based mimics of [Fe]-hydrogenase.


Introduction
8][9] Less studied is the 'third hydrogenase'namely the redox inactive [Fe]-hydrogenase (Hmd).The single iron site in this enzyme heterolytically activates H 2 and catalyzes hydride transfer to the C 1 carrier substrate methenyl-tetrahydromethanopterin (H 4 MPT + , Scheme 1), thus generating methylenetetrahydromethanopterin (H 4 MPT). 10The rened crystal structure reported by Shima in 2009 identied the active site environment, 11,12 and a 2019 report 13 described the crystallized enzyme in both the open (inactive) and closed (active, substratebound) conformations.The latter report precisely dened the proximity of the H 4 MPT + hydride transfer substrate to the iron center, and proposed detailed a mechanism of H 2 activation and hydride transfer using QM/MM calculations 14 based on the new protein crystal structures.Since 2009, researchers have signicantly advanced structural models of Hmd.However, the scope of functional mimics of Hmd remains limited.Hu and coworkers developed functional systems derived from hybrid moleculejprotein systems 15 and a small molecule system that incorporates an abiotic diphosphine ligand with a pendant amine base. 16Our group has reported model systems capable of hydride abstraction 17 (the enzymatic 'reverse' reaction) and hydride transfer 18 (enzymatic 'forward' reaction) with biomimetic substrates.However, both of our reported systems replicated the strong trans inuence of the Fe-C acyl s bond in the form of 'carbamoyl' ligation (i.e. -N H C ]O ) as a synthetically more accessible proxy for the endogenous methylene-containing acyl unit (i.e.-C H2 C ]O ); synthesis of the former was originally demonstrated by Pickett. 19,20Indeed, the preparation of acyl-containing synthetic systems that rigorously replicate the primary coordination sphere of Hmd and exhibit biomimetic reactivity has proven to be a particular challenge due the inherent instability of such compounds and their apparentand as yet unexplainedsensitivity to base.
In this report, we have more faithfully replicated the Hmd active site in comparison to our previous work by installing the biomimetic methylene linkage.Our synthetic approach uniquely uses an 'anthracene scaffold' that provides an accurate and stable means of emulating the biomimetic fac-CNS ligation motif.We rst describe the synthesis of the model complex and its well-described reactivity in the presence of base.We then demonstrate functional H 2 activation by a deprotonated ironacyl model complex that results in liberation of ligand and reduction of the Fe center instead of hydride transfer to a model substrate.Additional base in solution did, in fact, result in successful hydride transfer to the model substrate.However, through a series of control experiments we identify the active hydride transfer agent as the tetracarbonylhydridoferrate species, [HFe(CO) 4 ] À .Lastly, we describe a mechanistic pathway for reductive conversion of the Fe-acyl unit based on our observations from the structural (XRD, XAS, EXAFS) and spectroscopic ( 1 H/ 2 H NMR, IR) data collected.These observations provide clear benchmarks and 'warning signs' of false positives for other researchers working in the area of biomimetic [Fe]hydrogenase systems.
The 1 H NMR spectrum of 1 in d 8 -THF solution (Fig. S2 †) exhibits diamagnetic proton resonances with the characteristic methylene proton resonances observed as diastereotopic doublets at 3.97 and 4.52 ppm consistent with the ligation of the anionic acyl (-C H2 C ]O ) group to the iron center.The 13 C NMR under 1 atm 13 CO (Fig. S3 †) revealed the iron-bound carbon of the acyl moiety (d 254 ppm) to be exchangeable (t 1/2 z 3 d), while the 13 C^O ligands exchange slightly faster (t 1/2 z 2 d).Facile CO exchange of the acyl moiety was also reported in a complex reported by Hu. 21ttempts at isolation of single crystals of 1 were unsuccessful.Structural evidence supporting the core motif of 1 was obtained from the derivative complex bound with AsPh 3 .Addition of one equiv.of AsPh 3 to 1 enabled the isolation of single crystals of the closely related complex [(Anth$C H2 NS off ) Fe(CO) 2 (Br)(AsPh 3 )] (Fig. 1).The AsPh 3 adduct exhibits facarrangement of the C, N, As donor atoms, with the AsPh 3 ligand displacing the thioether-S ligand.The orthogonal face is Scheme 2 Ligand and metal complex syntheses.occupied by cis carbonyl ligands and the bromide is located trans to the acyl-C ligand as proposed in the structure of 1. Upon coordination of AsPh 3 , a small red-shi is observed in the n(C^O) stretches to 2024 and 1971 cm À1 and a notable blue-shi ($13 cm À1 ) to 1642 cm À1 is observed in n(C]O) stretch of the acyl unit (Fig. S22 †).Notably, the bound state of the original thioether-S in 1 was supported by XPS analysis (Fig. S36 †).

Methylene-acyl deprotonation by exogenous base
It is proposed that Hmd utilizes the pendant pyridonate-O as a proton acceptor to facilitate heterolytic cleavage of H 2 .Due to the absence of this basic functionality in the present ligand design, we previously reported 18 a system in which a bulky phenolate base, NEt 4 [MeOtBu 2 ArO], participated in H 2 activation to ultimately drive hydride transfer.We thus attempted the analogous H 2 activation in the presence of this base.However, in a synthetic scale reaction, treatment of 1 in THF with one equiv.NEt 4 [MeOtBu 2 ArO] immediately generated a red-orange solution, accompanied by a precipitate (NEt 4 Br).This contrasts carbamoyl-based systems (NH linkage, not CH 2 ), wherein no direct reaction with the same bulky phenolate is observed.Concentration of the ltered solution and successive washes with pentane and Et 2 O removed the protonated phenol byproduct (MeOtBu 2 ArOH), which was identied by 1 H NMR.
Extraction of the resulting powder into MeCN produced Xray quality crystals at À20 C. The resulting structure (Fig. 2) revealed a remarkable result: a dimeric complex in which two iron centers bridge via the formation of a new Fe-C bond between the deprotonated methine-C (formerly the methylene unit) of adjacent, identical units.The new dimeric species is formulated as [(Anth$C H NS off )Fe(CO) 2 (MeCN)] 2 (2).The bond distances of the new bridging Fe-C bonds are quite long at 2.186(6) and 2.194(6) Å.][24][25] Deprotonation of a methylene proton was also evident through shis in the IR spectrum and changes in the 1  The structure of 2 unequivocally conrms deprotonation of the methylene proton as proposed (but not unambiguously Scheme 3 Reversible deprotonation of 1 to form 2, and proposed bridging coordination of base.Note that the sequence to isolate 2 was performed in MeCN, while the sequence to examine the base-bridged dimer (far right) by EXAFS was performed in THF.proven) in another acyl-containing model compound (a mer-CNS dicarbonyl) recently published by our group, 26 suggesting that this mechanism is broadly applicable.Furthermore, deprotonation of the methylene-acyl moiety has been observed in another model compound by Song and coworkers through a suggested keto-enol tautomerization and acylation mechanism, although the analogous intermediate was not identied in that case. 27These observations suggest that this acyl moiety is rather reactive, and must be fully understood in structural and functional synthetic mimics of this enzyme.Indeed, exogenous base has been noted to decompose previous non-scaffolded acyl-containing model compounds, 16 perhaps related to this process.The scaffold-supported {Fe(CO) 2 } 2+ motif of complex 2, however, is stable and even accommodates further addition of base.

Bridging coordination of base to the Fe centers (XAS)
Treatment of 1 with two equiv. of NEt 4 [MeOtBu 2 ArO] in THF resulted in a more dramatic color change from orange to dark red.Additionally, the IR spectrum of the resulting solution exhibited further red-shied carbonyl stretching frequencies observed at 1996 and 1923 cm À1 (Fig. S24 †) in comparison to 1 or 2. The signicant red-shi is consistent with binding of the anionic phenolate donor to displace the Fe-C methine bonds.Coordination of bridging or terminal 2,6-di-tert-butylphenolates is not unprecedented in the generation of low-coordinate iron centers. 28,29The fully reversible nature of this event was demonstrated by treatment of the dark red solution with two equiv. of 2,6-lutidine$HBr to re-generate a solution of 1 as followed by IR spectroscopy (Fig. S25 †).
Attempts to determine the molecular structure resulting from the treatment of 1 with two equiv. of NEt 4 [MeOtBu 2 ArO] (or, equally, treatment of 2 with one equiv.of base) by X-ray crystallography were unsuccessful.The resulting species was thus probed by iron K-edge X-ray absorption spectroscopy (Fig. 4).The XANES region of the iron K-edge X-ray absorption spectrum displays a pronounced pre-edge peak at 7113.5(1) eV corresponding to a nominal Fe(1s / 3d) transition (Fig. 4A); the intensity of this peak is consistent with iron contained in a non-centrosymmetric coordination environment (e.g.5-coordinate distorted square pyramidal). 30The EXAFS data for 1 treated with two equiv. of base are best modeled as a dimer of ve-coordinate Fe centers ligated by two short CO ligands at 1.77 Å and three additional light atom ligand donors, modeled as N-scatterers, at 2.03 Å, which is similar to the two short carbonyl ligands (1.79 Å) and 3-4 light atom donors, modeled as N-scatterers, at 2.05 Å obtained from the model to the EXAFS data for 2. It is therefore likely that the three light-atom ligand donors modeled at 2.03 Å in 1 treated with two equiv. of NEt 4 [MeOtBu 2 ArO]are the acyl-C donor, a pyridine-N donor, and an additional coordinated phenolate-O donor.The Fe-CO bond length observed in 1 with two equiv. of NEt 4 [MeOtBu 2 ArO] is slightly shorter than the average Fe-CO distance observed in 2, and is consistent with the increased p-backbonding as corroborated by the red-shied carbonyl stretching frequencies.In addition to the Fe-CO signicant multiple scattering pathways found between R 0 ¼ 2.5-3.5 Å in the Fourier transform, which dominates the EXAFS of both 1 treated with two equiv. of NEt 4 [MeOtBu 2 ArO] and 2, an Fe/Fe vector could also be located.For 1 treated with two equiv. of NEt 4 [MeOtBu 2 ArO], the Fe/Fe vector is found at 3.44 Å; a wavelet transform of the EXAFS data of 2 clearly shows the Fe/Fe single scattering pathway is resolvable from the Fe-CO multiple scattering pathways, supporting this assignment (Fig. S42 †).In contrast, the XAS data for 2 yields an Fe/Fe single scattering pathway at 3.80 Å, which is consistent with the crystallographic results.Taken together, these data are fully consistent with the formulation of 1 with two equiv. of NEt 4 [MeOtBu 2 ArO] as a phenoxylbridged Fe/Fe dimer (Fig. 4).

Biomimetic H 2 activation by the rst dimer (2)
Complex 2 without base.Generation of 2 results in two analogous features of the Hmd active site: (i) a labile coordination site trans (MeCN) to the acyl unit and (ii) a basic site on the ligand.Notably, in contrast to the endogenous pyridone-O or PNP pincer complexes, 31 the location of the deprotonated methine-C basic site on the ligand framework trans to the open site is not positioned favorably for cooperative H 2 activation; nevertheless we hypothesized that the deprotonated 2 may still activate H 2 .A crystalline sample of 2 was dissolved in a THF solution containing model substrate [ Tol Im](BAr F ) as hydride acceptor and incubated with 7 atm D 2 .The 2 H NMR spectrum (Fig. 5A) of the reaction was monitored, revealing new resonances at 2.59 ppm and À14.90 ppm, corresponding to deuteration of the 2-methylpyridine moiety of the Anth$C H3 NS Me ligand and an Fe-D species, respectively.No hydride transfer product ( Tol ImD) was observed aer three days of monitoring.The isotopic inverse reaction (d 8 -THF, H 2 ) was performed with the free ligand and Fe-H species rst being observed aer 24 hours (Fig. S7 †).Incorporation of deuterium into the free ligand indicates that while 2 is competent for D 2 activation, D 2 activation and protonation of the methine-C results in the liberation of ligand from the {Fe(CO) 2 } unit.During this process, heterolysis of D 2 presumably results in the transient generation of the neutral species [(Anth$C HD NS Me ) FeD(CO) 2 ]; however, provided only the detection of the liberated Anth$C H3 NS Me ligand, we were initially unable to unambiguously ascribe the Fe-H or D resonance at À14.90 ppm.
Complex 2 with base.Provided our previous work, 18 we postulated that an extra equivalent of base in solution would drive H 2 activation and prevent protonation of the methine-C responsible for ligand loss.Therefore, 1 was rst treated with two equiv. of base (i.e.NEt 4 [MeOtBu 2 ArO]) and the model substrate [ Tol Im](BAr F ).The THF solution was incubated with 7 atm D 2 and the reaction was monitored by 2 H NMR spectroscopy.Two new resonances were observed in the 2 H NMR spectrum at 6.11 ppm and 5.57 ppm (Fig. 5B), corresponding to the successful hydride transfer product Tol ImD and MeOtBu 2 ArOD, respectively.Additionally, an unassigned peak at 2.21 ppm was observed that was distinct from the free Anth$C H3 NS Me ligand resonance.Attempts to optimize the desired hydride transfer reaction and suppress the peak at 2.21 ppm were unsuccessful.
Competitive formation of reduced Fe-carbonyl species H 2 activation without substrate (denitive reduced iron extrusion).To date, spectroscopic observation of a biomimetic Fe-H species capable of hydride transfer to an organic substrate has remained elusive in both Hmd enzyme and synthetic systems.To observe the putative Fe-H intermediate responsible for hydride transfer, we repeated the experiment in the absence of the substrate [ Tol Im](BAr F ) with the intention of trapping the reactive intermediate.A THF solution of 1 was rst treated with two equiv. of base (i.e.NEt 4 [MeOtBu 2 ArO]) and incubated with 7 atm D 2 .Indeed, the 2 H NMR spectrum exhibited two new resonances at 5.56 ppm and À8.87 ppm, corresponding to MeOtBu 2 ArOD and an Fe-D species, respectively (Fig. 5C).The isotopic inverse reaction (i.e.d 8 -THF, H 2 ) was carried out and the 1 H NMR displayed the analogous Fe-H resonance at À8.85 ppm within 1 hour of incubation (Fig. S9 †).The resulting 1 H NMR spectrum demonstrated a mixture of products over the course of the reaction, and we therefore attempted to more cleanly generate the Fe-H species through the use of the strong hydride donor, NaHBEt 3 .Again, in situ generated 2 treated with 0.9 equiv. of NaHBEt 3 resulted in a 1 H NMR spectrum displaying the same Fe-H resonance at À8.83 ppm (Fig. S10 †).
We serendipitously obtained dark red crystals from the THF solution of both the H 2 /D 2 and NaHBEt 3 reactions in the NMR reaction tube which werecontrary to our optimistic expectation -identied as the known di-iron carbonyl dianion (NEt 4 ) 2 [Fe 2 (CO) 8 ] by X-ray diffraction, proving the reduction of the ferrous starting material to Fe(À1).Provided the overwhelming evidence of reductive chemistry and our previous observance of unbound ligand, we considered a conversion pathway to better explain the formation of (NEt 4 ) 2 [Fe 2 (CO) 8 ] (Scheme 4) in the context of the observed Fe-H or D resonance and extrusion of the metal center from the anthracene scaffold.
We rst contemplated the retrosynthesis of the observed (NEt 4 ) 2 [Fe 2 (CO) 8 ] product, hypothesizing its derivation from bond formation between two simple {Fe(CO) 4 } building blocks.Upon inspection of known, simple iron tetracarbonyl compounds, we intuited that the product could be derived from initial protonation or deprotonation of one NEt 4 [HFe(CO) 4 ] unit, thus providing the necessary 2e À for the reduction of 2Fe 0 to 2Fe À1 , concomitant with generation of H 2 (i.e.Fe 0 -H + B / (Fe 2À + BH) + Fe 0 -H / 2Fe 1À + H 2 + B).Furthermore, the 1 H NMR resonance of the Fe-H of NEt 4 [HFe(CO) 4 ] was previously reported at À8.8 ppm (d 8 -THF), 32,33 which is obviously consistent with the Fe-H resonance (d H/D z À8.8) observed upon H 2 activation in our studies.To conrm this hypothesis, we independently synthesized PPN[HFe(CO) 4 ] (Fig. S11 and S26 †) according to literature procedure 32 and treated it with one equiv.of NEt 4 [MeOtBu 2 ArO] base to deprotonate the Fe-H species.Within minutes of base addition, we observed line broadening in the 1 H NMR spectrum (Fig. S12 †), consistent with reduction to form the intermediate paramagnetic Fe(À1) species concomitant with formation of a red precipitate, conrmed as (NEt 4 ) 2 [Fe 2 (CO) 8 ] by IR spectroscopy (Fig. S28 †).Indeed, PPN[HFe(CO) 4 ] is a known reductant 33 and the control experiment reacting independently synthesized PPN[HFe(CO) 4 ] and [ Tol Im](BAr F ) (Fig. S13 †) proved successful hydride transfer, thus strongly indicating NEt 4 [HFe(CO) 4 ] was the active hydride transfer agent in our previous experiments.Furthermore, at longer timepoints in this reaction (days), a new resonance at À14.79 ppm was observedsimilar to the previously observed, unassigned Fe-H/D species in Fig. 5A.We now conclusively assign this Fe-H species as NEt 4 [HFe 3 (CO) 11 ], a known side-product in hydride transfer reactions of NEt 4 [HFe(CO) 4 ]. 33ndeed, [ Tol Im](BAr F ) was separately treated with NEt 4 [HFe 3 (-CO) 11 ], but no hydride transfer reaction was observed over the course of several days (Fig. S14 †), further supporting the role of NEt 4 [HFe(CO) 4 ] as the exclusive active hydride transfer agent.
Identication of NEt 4 [HFe(CO) 4 ] also conrms the loss of ligand which was observed by 1 H NMR spectroscopy in both gas reactions utilizing H 2 (Fig. S15 †) and upon treatment with NaHBEt 3 (Fig. S16 †).Furthermore, we re-emphasize the observation of a feature at 2.51 ppm corresponding to deuteration of the methylpyridine moiety of the ligand in the 2 H NMR spectrum upon generation NEt 4 [DFe(CO) 4 ] (Fig. S8 †).
The liberation of ligand is predicated upon de-insertion of the acyl unit, which is capable of serving as a CO source in the generation NEt 4 [HFe(CO) 4 ].Upon de-insertion (Scheme 4, right side), the methyl carbanion coordinates to the Fe center to generate an intermediate related to that proposed in the synthesis of the acyl unit by Song and coworkers. 34These observations are also consistent with a less electrophilic CO ligand bound to Fe(0) in comparison to Fe(II) and the demonstrated lability of the acyl unit from labeled 13 CO exchange experiments. 21e investigated the reactivity of the proposed carbanion bound intermediate NEt 4 [(Anth$C H2 N off S off )Fe 0 (CO) 4 ] by independent synthesis of the lithium methyl-carbanion salt via lithiation of Anth$C H3 NS Me and addition of Fe(CO) 5 (i.e.omitting oxidation by Br 2 from the synthesis of 1).The IR spectrum of Li[(Anth$C H2 N off S off )Fe 0 (CO) 4 ] exhibited CO stretching frequencies of similar energy to the related complex described by Song 34 and to NEt 4 [HFe(CO) 4 ] and did not exhibit an n(C]O) feature above 1600 cm À1 , as would otherwise indicate acyl ligation (Fig. S29 †).We hypothesized heterolysis of H 2 across the Fe center and bound ligand could explain the generation of NEt 4 [HFe(CO) 4 ] and protonation to liberate the free ligand; however, no reaction was observed upon treatment of Li [(Anth$C H2 N off S off )Fe 0 (CO) 4 ] with D 2 by 2 H NMR spectroscopy (Scheme 4, bottom).Instead, treatment of Li[(Anth$C H2 N off S off ) Fe 0 (CO) 4 ] with two equiv.MeOtBu 2 ArOD indicated formation of D-labeled free ligand, Anth$C H2D NS Me , and NEt 4 [DFe(CO) 4 ] by 2 H NMR spectroscopy (Fig. S17 †).Analogous control experiments performed with 2,6-lutidine$HCl provided similar results, supporting that the phenolic proton was the active agentrather than H-atom or other radical chemistry.As indicated in Scheme 4, the extruded {Fe(CO) 4 } unit undergoes further chemistry to form NEt 4 [HFe(CO) 4 ]; however, the nature or mechanism of this particular reaction remains elusive at this time.
Lastly, we considered the initial reduction event of the ferrous starting complex to Fe(0).Based on the activation of H 2 / D 2 mediated by 2 and the control reaction treating 2 with NaHBEt 3 -and the spectroscopically detected reduced Fe carbonyl species-we postulate that reduction of the ferrous metal center occurs by loss of the unobserved, reactive hydride as a proton along with two electron reduction to form Fe(0). Consistent with our previous work, 18 detection of the highly reactive (especially anionic) Fe-H species of [Fe]-hydrogenase synthetic models is difficult.Intriguingly, this reductive pathway contrasts the well-characterized intramolecular hydride transfer reaction resulting in methylthiol extrusion observed in another model system from our group (mer-CNS; no scaffold), 35 likely due to the unbound state of the thioether-S Me unit downstream of 1 in this case.

Conclusions
In summary, we have prepared an acyl-containing anthracenescaffolded [Fe]-hydrogenase model compound that exhibits a dynamic fac-CNS donor motif and performs H 2 activation.The subtle structural replacement of the previously studied carbamoyl ligation for the methylene-acyl moiety provides a dramatically different reaction pathway to H 2 activation, which rst involves clean and structurally characterized deprotonation of the methylene linker.Notably, the anthracene-scaffolded model complex exhibits well-controlled reactivity upon base treatment in comparison to non-scaffolded systems, possibly due to the controlled hemi-lability of the thioether-S.The methine-ligated dimer 2 resulting from base addition was, itself, competent for H 2 activation, but hydride transfer to a biomimetic substrate was not observed.Instead, isotopic D-labeled gas experiments revealed formation of free ligand and the reductively extruded hydridoferrate species [HFe(CO) 4 ] À (which converts to [HFe 3 (-CO) 11 ] À over several days).The former species is unambiguously proven to be the active hydride transfer agent in the present study, while the latter species is more stable and thus ineffective for hydride transfer in this system.
Attempts to utilize exogenous base for H 2 activation in concert with 2 to prevent the loss of ligand and Fe reduction were unsuccessful, but importantly enabled us to structurally and spectroscopically characterize relevant intermediates in this process.Numerous control reactions delineate a mechanistic pathway describing these conversions.This enhanced understanding of this deleterious, competitive process may contribute to the design of a more robust biomimetic reactivity system for understanding the reactivity of acyl(methylene)containing synthetic analogues of [Fe]-hydrogenase.The inclusion of the authentic and biomimetic pyridone and/or thiolate motifs may drastically alter the reactivity prole(s) described herein, thereby providing more enlightened insight into Nature's delicate choice of donor identity and location in the [Fe]-hydrogenase active site.

General considerations
Commercially available reagents were used without further purication unless otherwise noted.Suppliers of relevant reagents are described in the ESI.† Solvents used for synthesis were procured from Fisher Scientic and dried over alumina columns using a Pure Process Technology solvent purication system, and stored over 3 Å molecular sieves until use; THF was stored over 3 Å molecular sieves and small pieces of sodium.High-pressure NMR tubes were purchased from Wilmad Labglass (Cat No. 524-PV-7).Infrared spectra were recorded on a Bruker Alpha spectrometer equipped with a diamond ATR crystal, all contained under inert atmosphere.UV/vis spectra were recorded on an Agilent Cary 6000i spectrometer.The 1 H, 2 H, and 13 C were collected using Varian Direct Drive 400 MHz, 500 MHz or 600 MHz instruments.X-ray diffraction and X-ray absorption instrumentation and experimental techniques are described in the ESI.† All cross-coupling reactions and syntheses of metal complexes were performed under N 2 atmosphere using Schlenk technique or glovebox conditions.
Ligand syntheses 5-(8-Chloroanthracen-1-yl)-2-methylpyridine (Anth$C H3 N$Cl).A mixture of 5-bromo-2-methylpyridine (2.02 g, 11.8 mmol), KOAc (3.43 g, 35.0 mmol), B 2 Pin 2 (4.43 g, 17.4 mmol), Pd 2 (dba) 3 (0.213 g, 0.233 mmol), and SPhos (0.194 g, 0.473 mmol) were prepared in 100 mL of dioxane under N 2 atmosphere inside a glove box.The reaction mixture was reuxed for 6 h, and the resulting orange color solution was used in a next step without isolation.In a separate vessel, 1,8dichloroanthracene (3.16 g, 12.8 mmol) was prepared in 20 mL of dioxane, and K 3 PO 4 (7.40 g, 34.9 mmol) was dissolved in 15 mL of degassed water.The anthracene solution and then the K 3 PO 4(aq) solution were added into the reaction solution.Aer reuxing for 12 h, the reaction solution was cooled to room temperature and ltered over Celite pad.The organic products were extracted with ethyl acetate (EA) and dried over Na 2 SO 4 .The product was further puried by silica gel column chromatography (7 : 1 to 4 : 1 hexane/EA) to afford a yellow solid.Yield: 2.07 g (58%). 1  2-Methyl-5-(8-(3-(methylthio)phenyl)anthracen-1-yl)pyridine (Anth$C H3 NS Me ).A mixture of 5-(8-chloroanthracen-1-yl)-2methylpyridine (Anth$C H3 N$Cl) (1.75 g, 5.76 mmol), 3-(methylthio)phenylboronic acid (0.967 g, 5.75 mmol), Na 2 CO 3 (0.610 g, 5.75 mmol), [Pd 2 (dba) 3 ] (0.105 g, 0.115 mmol), and XPhos (0.111 g, 0.233 mmol) was prepared in 160 mL of THF : H 2 O (7 : 1) under N 2 atmosphere.The reaction solution was heated at 85 C for 12 h under N 2 atmosphere.Aer cooling the solution to room temperature, the mixture was quenched with a saturated NH 4 Cl (aq) solution ($10 mL).The organic product was extracted with DCM and washed with saturated brine (2 Â 100 mL).The product was dried over Na 2 SO 4 and concentrated under vacuum, and further puried by silica gel column chromatography (4 : 1 to 1 : 1 hexane/EA) to afford a yellow solid.Yield: 1.58 g (70%). 1  (1).A portion of Anth$C H3 NS Me ligand (0.20 g, 0.51 mmol) was prepared in 15 mL of THF under N 2 atmosphere on the Schlenk line.Aer cooling the solution to 0 C, 1.6 M n-BuLi in hexanes (0.32 mL, 0.51 mmol) was dropwise added into the solution and stirred for 30 minutes.Next, the reaction solution was cooled to À80 C, and 67 mL (0.50 mmol) of Fe(CO) 5 (diluted in 5 mL of THF) was injected into the solution over 1 min.The solution was slowly warmed to À20 C while stirring for 3 h under dark conditions.In a separate ask, 26 mL (0.50 mmol) of Br 2 was diluted in 5 mL of THF under N 2 atmosphere.Next, the reaction solution was cooled to À70 C, and the Br 2 solution was dropwise added into the reaction solution.Aer stirring for 2 h at À70 C, the volatiles were removed under vacuum at room temperature.The residual solid was washed with pentane and Et 2 to afford an orange-yellow powder.Yield: 240 mg (77%). 1  [(Anth$CH 2 NS off )Fe(CO) 2 (Br)(AsPh 3 )].Compound 1 (40 mg, 65 mmol) and AsPh 3 (20 mg, 65 mmol) were stirred in 5 mL of DCM at room temperature for 2 hours then stored overnight at À20 C. The solvent was removed in vacuo, and the residual solid was extracted with Et 2 O.The Et 2 O soluble fraction was concentrated to afford a yellow-orange solid.Single crystals for X-ray diffraction were grown from vapor diffusion of pentane in to a vial of the complex dissolved in FPh at À20 C. Yield: 37 mg (62%). 1  [(Anth$C H NS off )Fe(CO) 2 (MeCN)] 2 (2).Compound 1 (0.050 g, 0.082 mmol) and [(2,6-ditertbutyl-4-methoxyphenolate)(NEt 4 )] (0.030 g, 0.082 mmol) were each separately dissolved in 5 mL THF and mixed.The THF solution of 1 turned red and a white precipitate [(NEt 4 )Br] formed upon mixing.The resultant solution was ltered over Celite and the solvent was removed by vacuum.The deep red residue was washed with pentane and Et 2 O to extract 2,6-ditertbutyl-4-methoxyphenol, affording a redorange powder.The powder was treated with acetonitrile to give a turbid red-orange solution which was placed at À20 C producing orange plates suitable for X-ray diffraction.Yield: 54.5 mg (62%). 1

Scheme 1
Scheme 1 H 2 activation and hydride transfer reaction catalyzed by Hmd (left) and active site's putative key intermediate in H 2 activation and hydride transfer (right).
H NMR spectrum resulting from base addition.The solution n(C^O) features in the IR spectrum of 1 (2021, 1956 cm À1 ) red-shied signicantly to 2005, 1947 cm À1 upon addition of base.The expected four n(C^O) features for the C 2 -symmetric dimer 2 are only observable in the ground crystalline sample at 2021, 1998, 1962, and 1943 cm À1 (Fig. S23 †).The deprotonation event (Scheme 3) resulting in generation of 2 was also achieved with weaker bases such as NEt 4 [p-BrtBu 2 ArO] or NEt 4 [p-CNtBu 2 ArO] but not NEt 4 [p-NO 2 tBu 2 ArO]underscoring the surprising acidity of this C-H bond.The deprotonation was clearly reversible upon addition of one equiv.of the weak acid Lut$HBr (2021, 1955 cm À1 ) (Fig. 3).This conversion was also evidenced in the 1 H NMR spectrum by disappearance of the characteristic diastereotopic methylene proton resonances of 1, and a new resonance at 4.45 ppm in 2.

Fig. 5 2 H
Fig. 5 2 H NMR spectra demonstrating (A) D 2 activation by 2 in the presence of model substrate [ Tol Im](BAr F ). Deuterium labelling is observed at 2.59 and À14.90 ppm, corresponding to the formation of D-labelled free Anth$C H2D NS Me ligand and [DFe 3 (CO) 11 ] À , respectively; (B) D 2 activation by 2 in the presence of an additional equivalent of base and model substrate [ Tol Im](BAr F ). Deuterium labelling is observed at 5.57 and 6.11 ppm, corresponding to the formation of Dlabelled MeOtBu 2 ArOD and Tol ImD, respectively; (C) D 2 activation by 2 in the presence of an additional equivalent of base.Deuterium labelling is observed at 5.56 and À8.87 ppm, corresponding to the formation of D-labelled MeOtBu 2 ArOD and [DFe(CO) 4 ] À , respectively.